The present invention relates to an electron microscope, and more particularly to an electron microscope which is improved in point of correction of focusing and/or astigmatism.
Optimum focusing and astigmatism correction are required in an electron microscope in order to observe and photograph an image of high resolution (an image of high magnification). However, manipulation, thereof is very difficult and requires skill. Desire for an effective automatic focusing and automatic astigmatism correction device has increased in recent years in order to reduce the burden on the operator, and several techniques have been proposed.
One of these techniques is to examine by utilizing covariance of an image obtained with an electron microscope to determine characteristics of the image as a function of focal aberration quantity or astigmatism quantity, thereby to conduct automatic focusing and automatic astigmatism correction (Journal of Microscopy, Vol. 127, Pt Z, August 1982, pp. 185-199). Further, another technique is to achieve a similar object utilizing the observation quantity of an image produced by inclining an irradiation beam (Ultramicroscopy 21 (1987) 209-222)).
In either case, however, practical use has been difficult particularly in a transmission type electron microscope because of detection sensibility, following problems in high magnification and so on. Namely, in a high magnification of a transmission type electron microscope, phase contrast becomes a problem and the configuration of a fine structure which is an object of observation is different depending on focal aberration quantity. Thus, since focusing while watching the frequency characteristic of an image is indispensable, it has been difficult to determine a proper focus point. In another example, a method of measuring aberration quantity of a focal point and astigmatism quantity by the result of applying two-dimensional Fourier transform to an image obtained with an electron microscope is presented (Micron 1981, Vol. 12, pp. 105-121). In still another example, a two-dimensional image is projected in one-dimension thereby to apply one-dimensional Fourier transform thereto for the purpose of improvement of processing speed and economization of a memory area in place of two-dimensional Fourier transform, which may be effective in focusing and astigmatism correction (Scanning Microscopy, Vol. 1, No. 4, 1987, pp. 1507-1514).
However, the above-described last two examples are confined to obtaining information such as focal aberration quantity and coefficient of astigmatism or to comparing an image with Fourier transform by applying two-dimensional or one-dimensional Fourier transform, and it is neither considered nor mentioned how a current of a focusing electron lens and a correction current of an astigmatism correction device are controlled in performing automatic focusing or automatic astigmatism correction.